Box 12 is informational. Since there isn't a dropdown for code TT currently, enter all of the other information from your W-2. Continue through the interview and you will be asked if you have any uncommon situations, including overtime. Select that you have the uncommon situation of overtime, then a series of questions will be generated.
On the screen "Let's see if your overtime qualifies as tax-free," select "No help needed—I know what to enter." You'll then be asked if you are an exempt or not an exempt employee (exempt employees are not eligible for the overtime deduction). Now, you can enter the deductible overtime.
Here is an article you may find helpful: No Tax on Overtime Explained: Qualified Overtime Deduction Rules for 2025